A bearing part or a machine structural part is processed into the final shape through the processing of a wire rod or a steel bar, such as scission, forging, or cutting. Particularly with respect to cold working (cold rolling or cold forging), a rolled material is so hard as it is that cold working is difficult. Therefore, the material is usually subjected to spheroidizing annealing prior to the cold working for the purpose of improving cold workability.
The ensuring of excellent cold workability is important also in terms of improving productivity and saving energy, thereby reducing cost and CO2 emission. In order to ensure excellent cold workability, it is necessary that deformation resistance is low, cracking does not occur during working, etc.
In addition, parts such as bearings and crank shafts are important parts that support rotating portions or sliding portions of machines. Because of the considerably high contact pressure and also of the varying external force, they are often used in severe environments. Therefore, steel materials, which are their raw materials, are required to have excellent durability.
In recent years, such demands are becoming severer year by year with the development of higher-performance and lighter-weight machines. For the improvement of the durability of shaft parts, although the improvement of techniques related to lubricity is also important, it is particularly important that a steel material has excellent abrasion resistance and rolling fatigue characteristics.
As a steel material used for bearings, a high-carbon chromium bearing steel, such as SUJ2 specified in JIS G 4805 (1999), has been conventionally used in various fields including automobiles, various industrial machines, and the like. However, such a steel material has high contents of carbon (0.95 to 1.1% by mass) and Cr (1.3 to 1.6% by mass), and coarse eutectic carbides, which adversely affect rolling fatigue characteristics, etc., are tend to be produced (e.g., precipitates of not less than 10 μm). In order to prevent the production of such coarse eutectic carbides, it has been necessary to perform, after blooming and before rolling, diffusion annealing at a high temperature (about 1250° C.) for a long period of time (about 17 hours), leading to high production cost. In addition, a bearing part produced from this steel material has been known to have a problem of insufficient rolling fatigue characteristics.
Under such circumstances, various techniques have been proposed as bearing steels.
For example, Patent Document 1 proposes a technique in which the contents of C (less than 0.6 to 0.95% by mass) and Cr (less than 1.3% by mass) are reduced, and also a specified content of B (0.0002 to 0.01% by mass) is included, thereby achieving the omission of the homogenization heat treatment and the shortening of the spheroidizing annealing time, so that the production cost is saved, and also excellent rolling fatigue characteristics and abrasion resistance are both ensured. However, this technique does not consider cold forgeability and may have a problem of cracking during cold forging, etc.
Meanwhile, Patent Document 2 proposes a technique in which the content of C (0.70 to 0.95% by mass) is reduced to shorten the diffusion annealing time, while a specified content of Sb (less than 0.0010% by mass) is included to improve rolling fatigue characteristics. However, this technique does not consider cold forgeability either and may have a problem of cracking during cold forging, etc.
Patent Document 3 discloses a patent in which after the spheroidizing annealing treatment, the average particle size of ferrite and the average particle size of cementite after cold drawing are specified to improve cold workability. However, because the C and Cr contents are high, and eutectic carbides may be produced, diffusion annealing is indispensable. Further, because 20 to 40% cold drawing is performed after spheroidizing annealing, the yield of the steel material decreases, resulting in increased production cost.